1. Field of the Invention
This invention relates to a process for forming doped regions beneath the surface of semiconductor substrates for the construction of integrated circuit structures therein. More particularly, this invention relates to a process for implantation of one or more dopants into a semiconductor substrate, using channeling to permit deeper implantation into the substrate, to thereby permit the formation of retrograde wells without the use of high energy during the implantation.
2. Description of the Related Art
Implantation of a dopant into a semiconductor substrate using a beam of charged atomic species is a well known method of introducing a dopant into a semiconductor lattice, for example, to change the semiconductor characteristics of the particular semiconductor material. Atoms of the dopant material are injected into the semiconductor lattice at a preselected energy level which generally controls the degree (depth) of penetration of the dopant into the semiconductor lattice. Subsequent to the implantation step, an anneal is carried out to repair damage to the crystal lattice caused during the penetration of the implanted atoms into the single crystal lattice of the semiconductor by the implantation.
Conventionally the depth of the implantation of the dopant into the semiconductor substrate is usually limited for a number of reasons, including the avoidance of further potential damage to the crystal lattice of the semiconductor substrate, the cost and availability of high energy implantation equipment, and the construction of integrated circuit structure close to the surface of the substrate, as well as other reasons.
Because of this desire to avoid deep implantations, the orientation of the crystal lattice of the semiconductor substrate is usually controlled, with respect to the axis of the implantation beam, to avoid having the implanted high energy atoms enter the crystal lattice at an angle which would permit the implanted atoms to pass between the atoms in the crystal lattice, rather than strike the atoms comprising the crystal lattice. Such a phenomena, known as "channeling", causes the high energy atoms to penetrate deeper into the semiconductor lattice since many of the implanted atoms do not encounter and impact the crystal lattice atoms, but rather pass between them.
FIG. 1 shows a prior art implantation setup wherein a silicon substrate 2, i.e., a silicon semiconductor wafer, has been tilted on axis 10 so that the cubic single crystal lattice of substrate 2 comprising silicon atoms 4 is not aligned with the paths of the dopant atoms 6 being implanted, i.e., so that implanted dopant atoms 6 are inhibited from passing down the paths or channels between the stacked silicon atoms in the cubic crystal lattice. By tilting the entire substrate 2, the dopant atoms 6 being implanted into the single crystal substrate 2 are more likely to strike the silicon atoms 4 in the crystal lattice of silicon substrate 2, in which case the dopant atoms 6 will not penetrate as deeply into substrate 2 as if they passed or "channeled" between silicon atoms 4 in substrate 2.
In certain circumstances, however, the use of a deeper implantation may be desirable, for example, for retrograde well formation to reduce latchup; or for forming deeper source/drain regions, under the same annealing conditions, to provide enhanced electrostatic discharge protection.
Formation of such a retrograde well, i.e., a well below the surface of the semiconductor substrate, or the formation of deeper source/drain regions, is conventionally carried out by increasing the implant energy for the particular dopant being implanted into the semiconductor substrate. However, such increased energy levels may necessitate the use of larger (higher energy) implantation apparatus, result in further damage to the semiconductor substrate being implanted, increase the lateral straddle of the implanted region (resulting in encroachment on adjacent doped regions of differing conductivity type or level), and result in gate penetration. It would, therefore, be desirable to be able to form doped regions beneath the substrate surface, such as retrograde doped conductivity wells in semiconductor substrates, by implantation without the need to increase the implant energy level of the implanted atoms.